1. Field of the Invention
The present invention relates to a process for producing a saccharified solution of lignocellulosic biomass.
2. Description of the Related Art
Conventionally, a process for producing ethanol by saccharifying lignocellulosic biomass, such as rice straw, as a substrate by a saccharifying enzyme produced by a microorganism and fermenting the yielded saccharides, has been known. The lignocellulosic biomass is constituted so that lignin is tightly bound to cellulose or hemicellulose. Consequently, for the saccharification is used a pretreated material for saccharification, which is the lignocellulosic biomass so pretreated that lignin contained in the lignocellulosic biomass is dissociated or the lignocellulosic biomass is swollen.
The term “dissociated” means herein that the bonds between lignin and cellulose or hemicellulose are at least partly cleaved. The term “swollen” means that a liquid penetrates between cellulose or hemicellulose constituting crystalline cellulose to generate gaps, or gaps are generated in a cellulose fiber, to expand the crystalline cellulose.
According to the conventional process for producing ethanol, the concentration of the substrate contained in the pretreated material for saccharification is often limited to a low level in order to reduce the consumption of the saccharifying enzyme, because the saccharifying enzyme is so expensive. Meanwhile, in case the concentration of the substrate contained in the pretreated material for saccharification is low, the concentration of the saccharified solution obtained from such a pretreated material for saccharification becomes also low, and consequently the concentration of the ethanol obtained by fermenting the saccharified solution becomes also low. As a result, there has been a problem that the time and thermal energy required for distillation in order to concentrate the obtained ethanol would increase.
To solve the problem, it is conceivable to increase the concentration of the substrate contained in the pretreated material for saccharification and the consumption of the saccharifying enzyme so as to obtain ethanol at a higher concentration. In this case, however, to cope with the cost increase due to the increase in the consumption of the expensive saccharifying enzyme, the total cost of the production process for ethanol needs to be curtailed.
One of the measures to curtail the cost in the production process for ethanol may be improvement of the efficiency of the saccharifying treatment of lignocellulosic biomass.
A process for producing monosaccharides such as glucose has been known, in which cellulose or hemicellulose constituting a cell wall of the lignocellulosic biomass is hydrolyzed using a saccharifying enzyme. By the process, monosaccharides, such as xylose, mannose and arabinose, originated from hemicellulose can be produced in addition to glucose.
For the improvement of the efficiency of the saccharifying treatment of lignocellulosic biomass, it is desirable to increase the yield of various monosaccharides including xylose in addition to glucose. In this regard, a combined use of a plurality of saccharifying enzymes in the saccharifying treatment of the lignocellulosic biomass has been known as effective (see e.g. Japanese Patent Application Laid-Open Publication No. 2009-171952).
There occurs, however, a disadvantage that the activity of a saccharifying enzyme, especially a saccharifying enzyme that hydrolyzes xylan to yield xylose, is inhibited in accordance with the increase in the produced amount of xylose, when lignocellulosic biomass is saccharified by the saccharifying enzyme. As a result, the yield of xylose is decreased, and the saccharifying treatment of lignocellulosic biomass cannot be improved sufficiently.